Could you be. Just for fun :p

>! **Shorts**?

You change me once, You'll find me on your phone or next to it.

>! A clumsy room mate throws shorts wherever he likes after removing it or changing it -even on phone or sometimes next to it

You change me twice, You'll find me on the sea or behind it.

>! There can be many scenarios - I'll leave up to reader's imagination :p

You don't change me, because you trust me.

>! Obviously we trust shorts that it will do its job of covering (and won't torn etc) :p

You can change me, because I trust you.

>! But yes you change also 

They said, those two things make me exist.

>! Shorts has two openings to easily insert your legs inside-two legs make it exist. It is made up of cloth and elastic. :p

___
**Title:**

My roommate's short riddle.
>! It is short - see **shorts** 

<br> <br> <br>
**Conversation:**

rm: Hi, good morning! I have a nice riddle for you.
>! **Morning** is a time to change shorts

me: seriously? Is this not too early?
>! **Describes** some prefer to not change shorts so early in the morning

rm: c'mon, just guess who am I :)
>! **c'mon**, is the *short* form of come on